Interpretive Summary: Rice is diploid with two sets of chromosomes (2n=2x=24, AA) and becomes aototetraploid with four sets of chromosomes (2n=4x=48, AAAA) when its chromosomes are doubled by a chemical, colchicine. Autotetraploid plants of most crops are very poor in pollen fertility and seed set percentage. Two F1 hybrids (D46AxDTP-4 and D46Ax Dminghui63) of auototetraploid rice (2n=4x=48) with extremely high pollen fertility 87.40% and 85.97%, respectively and seed set 82.00% and 79%, respectively were investigated for cytological behaviors in this study. More than 90% of the PMCs (pollen mother cell) formed normal microspores, which resulted in the production of viable pollen grains. Few abnormal microspores were observed including penta-fission and hexa-fission. Chromosome pairing of the two hybrids was normal at metaphase I (MI) with almost no univalents (I) or (trivalents) III, which are harmful for developing functional pollen. Chromosome configuration at MI was 0.02 I + 14.36 II (6.44 rod + 7.91 ring) + 0.01 III + 4.80 IV + 0.01 VIII for the former, and 0.06 I + 17.67 II (11.01 rod + 6.67 ring) + 0.06 III + 3.10 IV + 0.01 VI for the latter, respectively. The other meiotic phases progressed normally, except for low percentage of PMCs with both lagging chromosomes at AI and micronuclei at telophase I (TI) and telophase II (TII). PMCs with lagging chromosomes at AI and micronuclei at TI and TII were negatively correlated to pollen fertility and seed set. These results indicate that the two hybrids had better behaviors of chromosome pairing and genetic stability than other autotetraploid rice as well as other plants ever studied.

Technical Abstract:
Mechanisms of two F1 hybrids (D46AxDTP-4 and D46Ax Dminghui63) of auototetraploid rice (2n=4x=48) showing extremely high pollen fertility 87.40% and 85.97%, respectively, seed set 82.00% and 79%, respectively and extremely high level of heterosis were analyzed cytologically. Chromosome pairing of the two hybrids was normal at metaphase I (MI) with almost no I or III: 0.02 I + 14.36 II (6.44 rod + 7.91 ring) + 0.01 III + 4.80 IV + 0.01 VIII for the former, and 0.06 I + 17.67 II (11.01 rod + 6.67 ring) + 0.06 III + 3.10 IV + 0.01 VI for the latter, respectively. Bivalent frequency was less in hybrid than that in restoring parents, and the same results were gained for univalents, trivalents and multivalents. However, the quadrivalents frequency was significantly higher in hybrids than that in restoring parents at MI. The other meiotic phases progressed normally, except for low percentage of PMCs (pollen mother cell) with both lagging chromosomes at AI and micronuclei at telophase I (TI) and telophase II (TII). PMCs with lagging chromosomes at AI and micronuclei at TI and TII were negatively correlated to pollen fertility and seed set. More than 90% of the PMCs formed normal microspores, which resulted in the production of viable pollen grains. Few abnormal microspores were observed including penta-fission and hexa-fission. These results indicate that the two hybrids had better behaviors of chromosome pairing and genetic stability than other autotetraploid rice as well as other plants ever studied.